1. Field of the Invention
The present invention relates to manufacturing semiconductor devices; particularly to a method of forming a low dielectric constant film having improved film strength by inhibiting plasma damage, using plasma CVD.
2. Description of the Related Art
Film formation on a semiconductor substrate by plasma chemical vapor deposition (a plasma CVD method) is carried out by placing a semiconductor substrate, which is a workpiece, on a resistance-heating type heater a temperature of which is raised to 50-550° C., in 1-10 Torr atmosphere. By disposing the heater by pairing with a shower-plate jetting out a reaction gas, a plasma is generated by radio-frequency discharge between the heater and the shower-plate by applying radio-frequency power at 13.56-60 MHz at an output of 100-4,000 W to the shower-plate.
Using plasma CVD methods, thin film formation of interlayer insulation films, passivation films, reflection prevention films, etc. is carried out. As progress in ultrafine semiconductor devices is accelerated, in order to solve a problem of increasing RC delays, Cu wiring having excellent thermal durability and low resistance is used in place of conventional Al wiring. A dielectric constant of an interlayer insulation film decreases as device design rules becomes smaller; for devices in the 130 nm generation, SiOF films having a dielectric constant of approx. 3.4-3.7 are used. Since devices have become less than 100 nm, dielectric constants of interlayer insulation films have run in under 3, and low-k films (low-k silicon-containing films) are used. Additionally, for the purpose of Cu diffusion prevention, SiC, etc. began to be used.
For low-k films and SiC films, either of a coating method or a plasma CVD method is used. By contrast with the former, which does not have problems such as damage, etc., in the case of plasma CVD, a semiconductor substrate is electrically affected because plasma discharge takes place between upper and lower electrodes. For existing SiH4-based oxide films, and oxide films such as TEOS films which use a liquid source, it is easy to obtain a relatively stable discharge. As semiconductor devices become ultrafine, films required contain carbon, and a stable discharge area becomes smaller as compared with that in conventional arts; and depending on an internal state of a reaction chamber, overpolymerization is accelerated, and pulverization may occur instead of film formation. In connection with overpolymerization, unstable elements including plasma fluctuation are increasing.
There are two modes of plasma damage, leakage current which a surface charge on a substrate passes to the grounding potential through a susceptor heater, and lateral leakage current which depends on in-plane surface potential distribution. In order to improve an issue of plasma damage due to electric discharge, conventionally, methods of improving insulation by anodizing an electrode, increasing a thickness of anodization, and precoating an electrode prior to thin film formation were used for the purpose of improving electrode insulation. There are concerns about problems regarding these methods, however, including the necessity for process gas tolerance, plasma tolerance, and thermotolerance, and precoat film exfoliation. Recently, these problems are improved by using an alloy material having excellent corrosion resistance and tolerance. An example of this improvement is disclosed in Japanese Patent Laid-open No. 1999-229185.
Additionally, a method of improving plasma damage by increasing a pressure during film formation and decreasing a radio-frequency plasma amount to be applied was used. In this case, change in film quality caused by change in process conditions becomes a problem. When a plasma amount to be applied is increased, sheath voltage is increased, causing damage on a semiconductor substrate. There are methods such as a method of reducing the damage by controlling sheath voltage without changing a plasma amount to be applied, which is disclosed in Japanese Patent Laid-open No. 2003-45849.